Adipose cells grow rapidly over the first year of life, with the major increase in body fat being accounted for through growth in adipocyte size. By two years of age, however, obese infants manifest both increase in fat cell size and number. Careful study of the distribution of fat cell sizes has revealed marked differences when infants are compared. We wish to focus on the development of these differences in adipose tissue morphology in the first three years of life within a population of infants who are followed prospectively for assessment of preschol obesity status. The specific question which we will address relates to the extent to which differences in mean cell size, cell size distribution patterns, and cell number in the first three years of life are associated with concurrent differences in total body fat, anthropometric measurements of height and weight gain and fatfold thickness, parental body habitus, early infant feeding patterns, and subsequent obesity status at 36 - 48 months of age. The population to be studied will consist of those infants admitted to The Childrens Hospital of Philadelphia for elective hernia repair. A biopsy of abdominal subcutaneous tissue will be obtained at time of surgery and the children will be followed up at 6 - 12 month intervals through ages 3 - 4. Assessment of total body fat by anthropometric methods and by K40 determination of lean body mass will make it possible to estimate total fat cell numbers. This study will contribute to our knowledge of the extent of individual differences in adipose tissue growth in infancy, especialyly in relationship to both the known environmental and familial determinants of obesity and the the subsequent manifestation of obesity in the preschool period.